This invention relates to a proton exchange membrane (PEM) fuel cell in which porous reactant gas flow field plates have water flow fields on the reverse sides thereof which are dead-ended near the inlet of the corresponding reactant gas and which drain into the corresponding reactant gas exhaust manifold.
Water management in PEM fuel cells includes carrying the product water (resulting from the fuel cell process) and proton drag water (carried through the membrane from the anode side) away from the cathode, while at the same time ensuring that the membrane remains moist, particularly on the anode side. Recent designs of PEM fuel cells employ water transport plates which have reactant gas flow fields on one side and water coolant channels on the other side, to provide both the function of water management and the function of cooling the fuel cells. This requires a balance between promoting the flow of water through the water transport plate to humidify both reactants and to the anode electrode, to ensure that the membrane remains humidified, and flowing product and proton drag water away from the cathode electrode, while at the same time not allowing the water to block the passage of reactant gas to the respective electrodes. Furthermore, water flow between the cathode of one cell and the anode of an adjacent cell is provided by not using a gas-impervious separator plate, but relying on bubble pressure to prevent gas crossover between cells. This may be controlled by a careful balance of pore size and pressure differential between the reactant gases and the coolant water. However, the production of water transport plates with the proper size pores is extremely expensive. The utilization of larger pores for high water permeability while at the same time the utilization of smaller pores for high bubble pressure to prevent fuel reactant gas crossing over and mixing with the oxidant reactant gas, poses the dilemma; enhancement of one harms the other and vice versa. Fuel cells of this type are illustrated in commonly owned U.S. Pat. Nos. 5,503,944 and 5,700,595.
One problem with water transport plates, which utilize pure fuel cell water as the coolant, is that such fuel cells will freeze in cold climates, when not operating. Furthermore, the use of substances other than water in the water flow fields, to enhance thawing and initial startup, adulterates the fuel cell causing performance degradation.
Objects of the invention include the separation of water management from cooling in a PEM fuel cell; a PEM fuel cell which can use antifreeze solutions as a coolant; PEM fuel cells which can utilize start-up solutions to assist in thawing, when frozen; PEM fuel cells which do not require high tolerance, fine pore porous reactant flow field plates; PEM fuel cells employing easily manufactured, low tolerance plates for reactant flow fields and/or water management.
According to the present invention, passive water management in a PEM fuel cell includes oxidant reactant gas and fuel reactant gas flow field plates, at least one of which includes water flow fields on the reverse side thereof, the water flow fields being dead ended at ends thereof which are near the corresponding reactant gas inlet, and which flow into the related reactant gas exhaust manifold. In some embodiments, individual fuel cells of a stack are separated by gas-impervious plates; in one embodiment, at least some of the separator plates are cooling plates having coolant flow channels therein; in some embodiments, at least some of the separator plates are solid. The water management flow fields are thus at nearly the same pressure as the corresponding reactant gas exit manifold pressure; this pressure is lower than the pressure of the respective gas flow fields. The reactant gas pressure, in the reactant flow field, is maintained at a pressure which is substantially higher than the respective reactant gas exit manifold. The pressure differential is achieved by flow restrictions in the reactant gas flow field outlet. The pressure differential provides the driving force for moving excess water from the reactant flow field, through a porous or a perforated reactant flow field plate, into the water flow fields, and out to the reactant gas exit manifold, where it is removed from the cell. The pressure differential also sets the level of saturation of the anode and cathode substrates when wettable hydrophilic substrates are used.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.